I. Field of the Invention
The present invention generally relates to filter systems for the treatment of wastewater and potable water. In particular, the filter system of the present invention comprises a vessel containing multiple filter media situated in layers. The system utilizes a downflow filter technique including a periodic backflow process.
II. Background and Description of the Related Art
Home treatment plants are extensively used to treat wastewater discharged from houses which are far from city sewer systems. In fact, each minute, 4.2 million gallons of wastewater are discharged from home treatment plants in the United States alone. The effluent quality of wastewater from home treatment plants is evaluated using four parameters: dissolved oxygen ("DO"), pH, biochemical oxygen demand ("BOD") and suspended solids ("SS"). Treated wastewater must meet certain required acceptable tolerance ranges which are set forth in typical secondary effluent standards such as the Class I discharge standard of National Sanitation Foundation ("NSF") International. Usually, pH and DO requirements are easily achieved, and an upflow or downflow gravel filter is used as the last process to remove BOD and SS in some home treatment plants. However, many gravel filters are only backwashed after several months of service. Absent such backwashing, medial clogging problems can occur, resulting in inconsistent effluent quality. Similar filtering methods are used to treat effluent from commercial plants. However, poor maintenance of such systems also results in similar problems, leading to poor effluent quality.
In wastewater treatment applications in particular, the presence of suspended solids is frequently a major process problem. Filtration has commonly been employed to remove suspended solids from wastewater. Filtration normally occurs as the third step in a tertiary treatment process, following processing in a settling tank and a biofilm degradation process. Although traditional sand and mixed media filters are generally effective in removing suspended solids, the filtration bed is susceptible to clogging and high pressure drops across the bed. As a result, sand and mixed media filters require a manually controlled backwash procedure to clear accumulated solids.
Based upon the above problems, new types of filters have been developed, such as the filtration bag, the filtration sock, and polyurethane media and synthetic media filters. However, the filtration bag and sock require periodic replacement. Further, other filters are not suitable for home plants, which operate automatically and are inspected only once every six months. An invention that addresses the clogging problem is described in U.S. Pat. No. 4,906,381 to Barbaro, which discloses a fluid filtration unit comprising a number of filter modules. Each module has a pressure release valve. When a particular module becomes clogged with solids filtered from the fluid, the release valve allows that module to be bypassed and the fluid passes to the next unclogged module.
Recently, granular plastic media have been used as filtration media, but some types of plastic media are used only under gravity backwash conditions. If these media are used in a pump backwash system, the plastic media is coagulated into large particles by electrostatic force. This coagulation adversely affects backwash and filtration efficiencies.
Past attempts at filtering suspended solids in sand filters involved some manner of arranging the orientation of filtration media by placing the finer sand on the bottom and the larger grained sand on the top. This arrangement is contrary to that of a rapid sand filter, in which the finer grained sand is layered atop the larger grained sand. By using this arrangement in a downflow filter, much of the filter clogging experienced with the rapid sand filter can be prevented. The larger sand traps large solids, allowing finer particulate matter through to be filtered by the fine sand. The problem with such an arrangement has been that of maintaining the orientation of fine sand at the bottom and large sand at the top during backwashing. Upon backwashing, the larger sand particles tend to settle to the bottom so that resulting orientation is almost equivalent to the orientation of an ordinary rapid sand filter, that is, the larger grained sand settles to the bottom and the finer sand to the top. In practical application, anthracite particles and sand are used in double layer media filters because the specific gravity of the anthracite particles is less than that of the sand although the anthracite particles are larger. The anthracite particles therefore settle atop the sand after backwashing, and the desired media layering is maintained. The structure of the anthracite particles is too fragile to last through a long operation period, however. The problem of losing the anthracite particle media during backwashing is a significant disadvantage of this process.
Some of the above problems have been addressed by other inventions. For example, U.S. Pat. No. 3,814,247 to Hirs discloses a filter system using two granular filter media layers. The top media layer is composed of a course grained material. The layer beneath this is composed of a finer grained material having a specific gravity that is greater than that of the material above it. Synthetic as well as natural materials are disclosed as possible filter media for the upper layer. Backwashing of the lower layer is accomplished in the usual manner whereas the upper layer undergoes a more violent agitation and slurrying in a flow path external to the filter vessel. Frequent backwashing in this manner is performed in order to avoid a buildup of suspended solids at the face of the upper filtration medium.
U.S. Pat. No. 4,197,205 to Hirs discloses a filter system using a number of granular filter media layers. The top media layer is composed of a course grained material. The layers beneath this are composed of progressively finer grained materials having specific gravities that become progressively greater than that of the material above it. At least one layer is composed of a synthetic material. The layers are arranged in this fashion in order to reduce the occurrence of surface plugging.
U.S. Pat. No. 4,246,119 to Alldredge discloses an upflow or combination upflow/downflow filter system using a number of filter media layers. The filter media are compressed between two flexible diaphragms, one located above the filter media and one below. In the upflow configuration, finer filter media are layered on courser media. In the combination upflow/downflow configuration, a fine grained filter medium is packed between two courser media layers.
U.S. Pat. No. 4,692,248 to Stannard et al. discloses a filter apparatus which filters material from influent through a solid filter medium. A filter cake which forms on the surface of the solid filter medium is continuously washed away in the disclosed process utilizing the apparatus.
U.S. Pat. No. 4,851,122 to Stanley discloses a filter apparatus utilizing five layers of filtration media. The water to be treated first encounters an activated charcoal layer, used as a bacterial-reducing agent. Three successive layers of resin based media then remove impurities from the water. Finally, a layer of filtration medium such as crushed quartz, having a particle size much greater than that of the resin layers, removes sediment from the water.
None of the above inventions addresses nor solves the problems of filter efficiency or head loss over long periods of use. Head loss is the height difference between the water level in a fluid treatment plant and the treatment plant outlet. Head loss during the filtration cycle should be minimized for efficient water treatment. Initially, head loss during a filtration process is caused by the resistance of the filtration media themselves. As the media become clogged with impurities, it is this resulting sludge which increases the head loss of the system. Backwashing alleviates the head loss problem, but inefficient backwashing and clogged filter media following the backwash process increase head loss over the course of the wastewater treatment operation.
Those inventions which use synthetic filter media still have the problem of media coagulation following backwash. Loss of natural media through long term use and vigorous backwash continues to be a problem. The disclosed processes are not suited to automatic operation, especially operation including an effective backwash procedure, nor are the systems easily maintainable.
Faced with the foregoing difficulties in the application of conventional filters and new types of filters, the new multi-media filter of the present invention has been developed to improve filtration efficiency and head loss across the media, and eliminate the media loss problem.
It is therefore an object of the present invention to provide a filter for tertiary treatment of municipal wastewater which possesses the advantages of high filtration efficiency and easy maintenance.
It is a further object of the present invention to set up a media layer arrangement which has the largest grained filter medium on the top and the finest grained filter medium on the bottom, exactly the opposite of the rapid sand orientation.
It is another object of the present invention to use anti-electrostatic granular plastic media, which has a long practical life and does not affect filtration and backwash efficiency.
It is a further object of the present invention to provide an automatic filtration and backwash control system which does not need any manual control at all, and is inexpensive and simple to use.
It is yet another object of the present invention to provide a filter which will achieve the Class I discharge standard of NSF International.